GIS Data Models III GEOG 370 Instructor: Christine Erlien

Representing Geographic Space: Vector Data Structures Represent spatial locations explicitly Relationships between entities implicit –Space between geographic entities not stored

Vector Data Models Multiple data models –Examination of relationships Between variables in 1 map Among variables in multiple maps Data models –Spaghetti models –Topological models –Vector chain codes

Vector Data Model: Spaghetti Simplest data structure One-to-one translation of graphical image –Doesn’t record topology  relationships implied rather than encoded Each entity is a single piece of spaghetti PointLineArea very short longer collection of line segments –Each entity is a single record, coded as variable- length strings of (X,Y) coordinate pairs –Boundaries shared by two polygons  stored twice

Vector Data Model: Spaghetti From Fundamentals of Geographic Information Systems, Demers (2005)

Vector Data Model: Spaghetti Measurement & analysis difficult –All relationships among objects must be calculated independently Relatively efficient for cartographic display –CAC Plotting: fast

Vector Data Model: Topological Topology: Spatial relationships between points, lines & polygons Topological models record adjacency information into data structure –Line segments have beginning & ending Link: Line segment Node: Point that links two or more lines –Identifies that point as the beginning or ending of line –Left & right polygons stored explicitly

Vector Data Model: Topological From An Introduction to Geographic Information Systems, Heywood et al. (2002)

Topological Data Models Multiple models –GBF/DIME (geographic base file/dual independent map encoding) –TIGER (topologically integrated geographic encoding and reference system) –POLYVRT (POLYgon conVERTer)

Topological Data Models: GBF/DIME Created by U.S. Census Bureau –Both street addresses & UTM coordinates defined for each link –Topology + direction Left/right From/to From Fundamentals of Geographic Information Systems, Demers (2005)

Topological Data Models: TIGER Designed for use with the 1990 U.S. Census Block-level maps –Points, lines & areas explicitly addressed –Census blocks can be retrieved directly by block number Area Line Point Coordinates From Fundamentals of Geographic Information Systems, Demers (2005)

Topological Data Models: POLYVRT From Fundamentals of Geographic Information Systems, Demers (2005) Entities stored separately but linked to one another through pointers Chains: Collections of line segments with directional information (from-to, left/right)

Shapefile Comprised of 3 file types *.shp contains coordinates *.shx is an index file *.dbf is an attribute file in dbase format Where is topology? Not explicitly stored Created on-the-fly Shapefile  in-between spaghetti & topological data structures ArcGIS/Arcview’s shapefile

Compacting Vector Data Models Compact data to reduce storage Freeman-Hoffman chain codes –Each line segment Directional vector Length –Non-topological Analytically limited  limits usefulness to storage, retrieval, output functions –Good for distance & shape calculations, plotting

TIN allows us to record topographic data as points in a regular or irregular grid. Vector Model to Represent Surfaces: TIN

From Geographic Information Systems & Science, Longley et al. (2005) Vector Model to Represent Surfaces: TIN

Vector GIS: Hybrid & Integrated Systems Hybrid system –Links graphic data structures with DBMS Efficiently manage both graphics & attribute data Allows raster & vector data types From Fundamentals of Geographic Information Systems, Demers (2005)

Vector GIS: Hybrid & Integrated Systems Integrated system –Entities’ coordinate data stored as relational table –Topological data stored as separate table in same database –Attributes can be Stored in same tables as graphic entities Stored as separate tables & linked relationally –GIS more closely integrated with DBMS than in hybrid system

Vector GIS: Object-Oriented Database Management Systems Emerging as an alternative to hybrid or integrated models Extends the integrated model by incorporating a spatial query language Objects inherit properties from the class of objects that they belong to –Variable types & operations particular to that class Example: ArcGIS’ geodatabase –Shift from previous hybrid orientation